In recent years, an Internet has been popularized in home, and high-speed and cheap communication services, in which a constant connection has been made by ADSL or FTTH, of which a transmission speed is several Mbps, and of which a monthly subscription is several tens of dollars, have been provided. Further, with the spread of broadband communication, a large amount of contents have been distributed, and a video chat has come into wide use.
With the spread of broadband communication, a B-PON system capable of providing high-speed data communication having a data transmission speed of 100 Mbps at a peak time and of simultaneously distributing multi-channel (for example, a maximum of 500 channels) image signals using one optical fiber has been used as an international standard of ITU-T. The structure of this system is shown in FIG. 26 (for example, see an outline of the B-PON system described in the home page of an NTT-AS institute).
Further, in FIG. 26, a communication station 40 is connected to a user home 41 through a communication optical fiber 45. In FIG. 26, B-ONU indicates a data-based subscriber station apparatus for data, WDM indicates an optical coupler, V-ONU indicates an image-based subscriber station apparatus, NE-OSS indicates a data-based monitor control apparatus, B-OLT indicates a data-based service station apparatus, EMDX indicates an Ether demultiplexing apparatus, and V-OLT indicates an image-based service station apparatus.
The salient characteristic of this system is that one optical wavelength is added to two wavelengths used for the transmission and reception of high-speed data by an additional optical wavelength arrangement and thus the distributed multi-channel images can be simultaneously seen. In general, wavelengths of high-speed data communication signals to be transmitted and received are included in a wavelength band of 1.49 μm (1.48 μm to 1.50 μm) or a wavelength band of 1.31 μm (1.26 μm to 1.36 μm), and an image is distributed at a wavelength band of 1.55 μm (1.55 μm to 1.56 μm), which is an amplifying band of EDFA.
Since optical signals having three types of wavelengths are transmitted through one optical fiber, the B-PON system requires a wavelength multiplexing and demultiplexing function having an optical signal multiplexing and demultiplexing function (the multiplexing and demultiplexing of optical signals having a plurality of wavelengths) and a function of distributing image signals with the same intensity. Conventionally, a dielectric multi-layer filter has been used as an optical component having the wavelength multiplexing and demultiplexing function. An optical component having the wavelength multiplexing and demultiplexing function requires a broadband wavelength multiplexing and demultiplexing characteristic and a high-isolation characteristic greater than, for example, 25 dB, and a development of such a broadband wavelength multiplexing and demultiplexing filter is demanded.
Further, since optical signals having three types of wavelengths are transmitted through one optical fiber, the B-PON system requires an optical signal multiplexing and demultiplexing device having an optical signal multiplexing and demultiplexing function (the multiplexing and demultiplexing of optical signals having a plurality of wavelengths) and an optical splitter for splitting image signals at the same intensity.
Accordingly, an object of the present invention is to provide a broadband wavelength multiplexing and demultiplexing filters improved for or capable of meeting the above-mentioned requirements. In addition, another object of the present invention is to provide an optical splitter with an optical signal multiplexing and demultiplexing function, which is suitable for a B-PON system, etc.